The present invention relates to electrographic printing and copying, and more particularly to a novel method and apparatus for duplex electrographic imaging.
In the broad arts of electrographic printing and copying considerable effort has been expended in designing systems for transferring toner images to both sides of a receptor medium. This is commonly known as "duplex imaging". Prior art processes include sequential duplex imaging, wherein toner images are formed on opposite sides of the receptor sheet in sequence, and simultaneous duplex imaging, wherein the two images are simultaneously transferred. The need for duplex printing and copying has long been recognized but has encountered various handicaps to effective implementation.
The various techniques for duplex imaging may be classified according to the method employed to transfer the toner image to the ultimate receptor, and perhaps as to intermediate transfers. Typical techniques include electrostatically aided transfers, transfers requiring heating the toner image, transfers using solvent vapors, and transfers requiring high pressure. Where a process as conventionally performed utilizes heat or a solvent vapor to cause the toner image to become soft and tackified, this poses a problem of offsetting and smudging in duplex imaging, particularly where both sides of the sheet are subjected to heat or vapor simultaneously. Alternatively, when sequential transfer has been employed, long time delays have been required to assure complete fusing of the first image prior to transfer of the second. It is well known in addition that heat fusing causes wrinkling and drying of the paper.
U.S. Pat. No. 3,318,212 discloses transfer imaging systems involving a plurality of support structures, using solvent vapors to tackify the toner image. In this system, the toner images are simultaneously transferred to opposite sides of the receptor sheet. This does not avoid the inherent liabilities of solvent vapor toner transfer systems, such as the loss of solvent vapors by evaporation, and the danger of flammable or toxic vapors. The presence of such vapors imposes various demands in machine design which considerably complicate such devices.
U.S. Pat. No. 3,697,171 discloses a simultaneous image transfer system wherein a toner image is formed on a first roller, electrostatically transferring to an intermediate roller, a second image then formed on the first roller, followed by simultaneous electrostatic transfer of two images to a receptor medium passed between these two rollers. Again, this system requires postfusing, and suffers the limitations inherent in electrostatic toner transfer, such as image density losses due to toner transfer inefficiencies.
Accordingly, it is a primary object of the invention to provide a reliable duplex electrographic imaging technique. In particular, this technique should provide a simultaneous transfer and fusing of the toner image.
Another object of the invention is to design duplex imaging apparatus providing high quality transfer and fusing of toner images. As a related object, both images shall be well defined, with little or no smudging or mottling. A further related object is that such images be permanently bonded to the carrier, without need for postfusing.